Polymeric dialdehyde-protein adhesives and wood laminates therewith



United States Patent C) F 3,153,597 PGLYMERKQ DlALDEHYDE-PRQTEW ADHESIVES AND W001) LAMINATES THEREWITH Francis l3. Weakiey and Charles L. Mehitretter, Peoria,

1th, assignors to the United States of America as represented by the Secretary of Agriculture N Drawing. Filed Nov. 16, 1961, Ser. No. 152,950 3 Claims. (Cl. 106-139) (Granted under Title 35, US. Code (1952), sec. 266) A nonexclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This invention relates to greatly improved plywood and similar laminated wood products. More particularly, this invention relates to superior plywood and related lami-- nated wood products that retain very high shear strengths even after being subjected to severe weathering as mea ured by resistance to boiling water. Still more particularly, this invention relates to exceptionally durable plywood and laminated wood products that owe their resistance against loss of shear strength after weathering as well as their possession of other valuable properties including a nonstain glue line to a novel self-curing glue prepared by reacting a polymeric dialdehyde such as commercially available periodatecxidized dialdehyde starch (containing about 90 percent or more of theory of aldehyde groups) or dialdehyde cellulose with an animal protein such as casein or a vegetable protein such as soybean pro tein in certain essential and critical proportions.

This application i a continuation-in-part of copending application Serial No. 69,237, filed November 14, 1960. In the said copending application we have disclosed the preparation of novel paper coating and sizing compositions that resemble the improved wood glue products of the instant application in employing the same protein and polymeric dialdehyde reactants. However, these paper coating and sizing products of the parent application are prepared by reacting the protein at room temperature with l020 percent of the polymeric dialdehyde, based on the dry weight of the protein, to form highly specific proteinpolymeric dialdehyde copolymers, mildly acid or alkaline coating composition dispersions of which then cure at room temperature to become irreversibly insolubilized, flexible copolymers that are highly resistant to the action of acidic, neutral, and alkaline solutions and confer these values to the paper treated therewith. However, a relatively high proportion of polymeric dialdehyde is required for complete insolubilization of casein at room temperature and economically low proportions of dialdehyde starch were not as effective. It was thus entirely unexpected that such low proportions of dialdehyde starch in dialdehyde starch-casein mixtures could be employed as cold-setting and self-curing glue, particularly for wood laminates for outdoor use wherein the latter are subject to constant weathering and to high shear strains.

In distinction from the paper sizing agents of the said copending parent application, we have now further discovered that we can prepare wood glues that are effective as bonding agents for both indoor and outdoor wood veneers or laminates by reacting a protein such as casein or soybean protein with between 2.5 percent and 5.0 percent of periodate oxidized starch containing at least 90 percent of theory of aldehyde groups.

The structural advantages of first quality plywood and edgeand end-glued laminates bonded with presently available nonstaining glues has resulted in their widespread indoors use. However, such glues are not sufficiently weather resistant to allow their use in the production of plywood and laminates for outdoor construction which is 3,153,597 Patented Oct. 20, 1964 a greatly expanding industry. On the other hand, glues used for exterior plywood are not generally satisfactory for bonding plywood for interior decorative purposes because of the dark glue line produced. For rapid production of such plywood the glue must also have rapid tack and good-green bond strength in addition to good dry bond strength. Phenolic polymer such as the phenol-resorcinol resins are widely used as adhesives in plywoods for exterior use, but plywoods bonded with the last mentioned adhesives are not recommended for interior use because of their marked staining tendency.

Casein glues are very well known, but until now it has been necessary to react casein with alkali or lime to obtain good glue strength. However, when such casein glues are used in manufacturing wood laminates, staining of the wood at the glue line occurs, and the glues are not sufliciently resistant to the weathering eiiects of continued outdoor exposure to permit Wood laminates comprising the same to be employed in outside locations. Casein has also been insolubilized with glyoxal or with the lower aliphatic aldehydes such as formaldehyde. However, the glyoxal-casein adhesives are darkly colored, and the necessary handling of the irritant and somewhat toxic volatile aliphatic reagents is rendered particularly unpleasant to the workmen. Furthermore, loss by volatilization requires the use of a large excess during application.

Accordingly, a principal object of the instant invention is the provision of a nonvolatile, nonirritating self-cross linkingor insolubilizing agent for protein adhesives. Another object is an improved, waterproof, cold-setting, wood glue which can be used to prepare wood laminates that do not strain at the glue line and that retain their original very high shear strengths despite prolonged weathering, and thus are suitable for preparing wood laminate regardless of where the laminates are to be used. A further object is the preparation of an aqueous borax dispersion of casein at slightly acid to slightly alkaline pH to which dispersion is added powdered dialdehyde starch with mixing just before spreading of the final glue composition on the wood panels to be laminated. This procedure naturally includes the expedient of coating the wood panels with the aqueous borax dispersion of casein followed by the.

uniform addition of the powdered dialdehyde starch'to the surface of the casein layer prior to assembling and cold pressing the panels to form laminates. A more specific further object is the preparation of a waterproof, coldsetting glue from casein and dialdehyde starch, it being recognized in this connection that a low degree of heatcuring may sometimes beadvantageous, particularly when the moisture content of the veneers after gluing and conditioning' is high. The above and related objects of this invention will be made clearer in the course of the following disclosure.

Briefly, in accordance with the objects of the instant invention, we have discovered that an aqueous dispersion of dialdehyde starch (starch that has been substantially oxidized by periodic acid so that the oxidized starch polymer contains at least percent and preferably at least about percent of theory of aldehyde groups and consisting essentially of an alternating series of hemiacetal-linked glyoxal and erythrose units), can be reacted at room temperature with an aqueous dispersion of casein, at a pH of between 6-8 (preferably about 6.3), the ratio of dialdehyde starch to the protein being 2.5-5.0 percent, based on the dry weight of protein present. A lowering of the pH retards the reaction and permits a longer pot life whereas raising the pH accelerates the reaction and may produce premature gelation. At pH 7.8 the reaction is so rapid that the product must be used instantly and the plywood must be made on special machines to avoid loss by gelation.

The following specific examples are set forth to more fully describe the practice of the invention.

4 EXAMPLE 2 A wood glue was prepared as in Example 1 excepting that component A thereof was adjusted with sodium hy- EXAMPLE 1 droxide to a pH of 7.8. Plywood panels prepared as in Component A Was prepared by slowly mixing 15 parts 5 Example 1 were conditioned and then dr ed at 140 by Weight of casein and 2 parts by Weight of borax in for 15 minutes before testing. The data is presented in 83 parts of water and heating to 80 C. for 30 minutes Table T bl H with continuous mixing. 'After cooling to 25 0, Water a 9 lost by evaporation was replaced. The solution had a 10 Brookfield viscosity of about 12 poises and a pH of 6.3. Dry shear Wei Shear Strength: Component B was prepared by adding 5 parts by weight Plywood Treatment strength, of commercial dialdehyde starch (93 percent dialdehyde gggf; $8 content) to 45 parts of water at 60 C., stirring for a few test minutes, adding 0.5 part by Weight of borax, raising the 15 temperature to 73 C. and continuing the stirring until 2. 381 1s1 12s. A the dialdehyde starch dissolved. After cooling to 25 Casein control 158 delaminated ddammahd' C. the soluion for convenience was brou ht u to 50 arts by volumeL a p P Additional plywood panels were also sub ected to the T116 copolymeric caseimdialdehyds Starch Wood glue cold soak for delammatron test in WlllCh the panels adhesive was Prepared by mixing 100 parts by volume of are submerged in water at room temperature for hours cgmponent A (15 parts by Weight of casein) and 375 and then dried at 24 -27 for 20 hours, repeating the parts by volume of component B (0375 part by Weight cycle until the speclmens fail or 15 cycles are completed. of DAS) (a 49:1 ratio of casein to DAS) at room The average score for 10 panels was 13 cycles, thus experature until a uniform, smooth, nearly colorless, highly Caedmg mmlmum Satisfactory score by 30 Percentfluid paste was obtained having a pH of 6.3 and an initial EXAMPLE 3 viscosity of about 12 poises at 25 C. 10

0 parts by volume of com onent A of Example 1 The above product was spread in an amount equal to T '47 lbs. per 1000 sq. ft. of glue line on the facing surfaces s g g r1 31 g gi i i g hiidgoxlde of each of two birch outer veneer panels and on 30 part. y t W m i i was both surfacss of Center p y Banal and the membars mixed therewith, making an actual weight ratio of 0.75 sandwiched at 100 lbs psi at room temperature for of the polymengglaldehyge to i s Welght a o casein or one to parts y werg t. e mixture 3?: g2; g ig gg i gig ggg sg g gg gg g fi: had an initial viscosity of 28 poises at 25 C. which inl e n midity before testing for dry shear strength. Wet shear 35 gg i g 5 2 gg s z g ggg Pa 3 22 2 strength was measured after soaking in water for 48 hours tioning alone (designated as Plywood Treatment 1) or as ywoo reatment 2 were su jected to ry s ear m Whlch the Plywood condltloned as above deslgnated and to the Water boil and dry cyclic test with the results as Plywood Treatment 1. 40 shown in Table In Plywood panels employing the same adhesive were Table In prepared as were the preceding panels but in addition they were then dried at 140 C. for 15 minutes after the Dry Shear Wet Shear previous conditioning step. No glue line was seen even Plywood Treatment strength, strength, psi. in the panels subjected to the drying step. The plywood 5 fig members thus treated and designated as Plywood Treatment 2 in Table I were subjected to the same accelerated 345 23 tests as were the preceding panels. Control plywoods pre- 65 76. pared with casein only (component A) delaminated while 0mm 00mm] 2451 (395)" delammated' lsagginggtgut for testing or showed substantially no shear lplywood Treatment 2' i Table I Table "IV shows the results of the cold soak for delamination test.

Table IV Wet shear strength, p.s.i. N b

- t Plywood Treatment tDrytfihean C t W t b 1 Number ofcyoles passedper specimen 533113 111 en .1. On lnllOllS 8. er 01 S r g p S immersion and dry pane S cyclic test 1 er 1 324 97. 55. 13:1: 31 2 399.. 177 105. 1% 5 Casein control (A) delaminated delaminated..- delaminated.

v v If the average number of cycles passed by the 10 panels ;gtgiifigfi iflmifi g itjfgfijgfiifi? E3355 gf g itggfi d 6 is less than 10 they are classified as defective. The

.test panels are immersed in boiling water for 4 hours and dried for 20 hours at 62 C. They are then boiled again for 4 hours, cooled in Water and the shear strength determined while wet.

' average per panel for the above 10 panels was 13 and thus exceeds the requirements of the test. Calculation of this average is as follows:

5 X 1: 5 12 1= 12 13 3= 39 15 X 5-: Total cycles=131 Total specimens 10 =13l average number of cycles per panel The results shown in Tables III and IV indicate that the dry and wet shear strengths of plywood glues with caseindialdehyde starch at pH 7 .8 are about the same as the value obtained at pH 6.3 after drying at 140 C. for minutes.

EXAMlLE 4 In the same manner in which we prepared the casein containing component A of Example 1, we separately prepared corresponding A solutions containing respectively 12.5 parts by weight per 100 ml. and 14 part by weight per 100 ml. of soybean alpha protein. To each of the respective 100 ml. component A solutions Was added 3.5 ml. of the component B solution of Example 1, corresponding respectively to 2.8 percent and to 2.5 percent of dialdehyde starch, based on the dry weight of the soybean alpha protein, and the pH of each was adjusted to a value of 7.8. The resulting glue was applied as in Example 1, and the plywood panels were compressed at 150 lbs/sq. in., after which they were conditioned in accordance with the two modifications shown in EX- ample 1 before subjecting them to test, the results of which are shown in Table V and Table VI.

1 Plywood Treatment 2.

5 Table VI 12.5% Soybean alpha protein Wet shear strength, p.s.i. Plywood Treatment Dry shear strength p.s.i. Continuous Water boil immersion and dry cyclic test 367 208 168. Protein control (2) 177 58 delaminated.

1 Plywood Treatment 2.

Having disclosed our invention we claim:

1. As a composition of matter, a nonstaining, highly water-resistant, self-curing general purpose plywood glue comprising the room temperature reaction product of an aqueous borax-dispersion containing 100 parts of a protein selected from the group consisting of casein and soybean alpha protein and an aqueous borax-dispersion containing 2.5 to 5 parts, based on the dry weight of the protein, of dialdehyde starch containing at least about percent of theory of aldehyde groups, said reaction product having been formed at a pH of about from 6.0 to not above 7.8.

2. The composition of claim 1 wherein the protein is soybean alpha protein.

3. The composition of claim 1 wherein the protein is casein.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AS A COMPOSITION OF MATTER, A NONSTAINING, HIGHLY WATER-RESISTANT, SELF-CURING GENERAL PURPOSE PLYWOOD GLUE COMPRISING THE ROOM TEMPERATURE REACTION PRODCUT OF AN AQUEOUS BORAX-DISPERSION CONTAINING 100 PARTS OF A PROTEIN SELECTED FROM THE GROUP CONSISTING OF CASEIN AND SOYBEAN ALPHA PORTEIN AND AN AQUEOUS BORAX-DISPERSION CONTAINING 2,5 TO 5 PARTS, BASED ON THE DRY WEIGHT OF THE PROTEIN, OF DIALDEHYDE STARCH CONTAINING AT LEAST ABOUT 90 PERCENT OF THEORY OF ALDEHYDE GROUPS, AND REACTION PRODUCT HAVING BEEN FORMED AT A PH OF ABOUT 6.0 TO NOT ABOVE ABOUT 7.8. 